1. Field of the Invention
The present invention relates generally to an internal combustion engine exhaust system and more specifically to an exhaust gas purifying device for reducing particulate matter emissions.
2. Description of the Prior Art
FIG. 1 shows an exhaust system which is disclosed in JP-A-58-51235 and which includes a trap for removing particulate matter (e.g. minute carbon particles) from the exhaust gases before they are released into the ambient atmosphere.
In this prior proposed arrangement the particulate matter which is contained in the gases exhausted from the combustion chambers of an internal combustion engine 1 into an exhaust conduit 2, are collected in a trap 3. This trap includes a hear resistant filter element (not shown in this figure) which separates the particulate matter from the gases content of the engine exhaust.
The engine includes an induction passage 5 in which a butterfly type throttle valve 6 is disposed. A lever 7 is connected to the shaft of the valve 6 and operatively connected with a diaphragm type vacuum motor 8 by way of a link 8a.
A solenoid valve 9 which controls communication between a vacuum pump 10 and a vacuum chamber 8b of the vacuum motor 8 is operatively connected with a control unit 15. This latter mentioned unit is connected with a fuel injection pump 11 and arranged to receive a load indicative signal produced by a load sensor 13 and an engine speed sensor 13. In this instance both of the sensors are associated with the pump 11 as shown. The control unit 15 is also connected with an induction pressure sensor 14 in a manner to receive a signal indicative thereof.
The control unit 15 is arranged to determine the timing with which the trap 3 should be regenerated based on either time or distance travelled. Upon such a determination being made, the control unit determines if the engine is operating in a predetermined engine speed/load range by selectively sampling the outputs above mentioned sensors.
Given that the engine is operating in the predetermined speed/load range, the control unit issues a signal to the solenoid which induces the throttle valve to partially close. The degree to which the throttle valve is closed and induction is throttled is feedback controlled based on the output of the induction pressure sensor 14. This feedback control is such as to adjust the duty cycle of the solenoid driver signal in a manner to establish an essentially constant negative induction pressure in the induction manifold downstream of the throttle valve 6.
When the amount of air which is inducted into the engine is reduced in this manner, the temperature of the exhaust gases is increased, the temperature of the trap rises and the particulate matter collected in the trap 3 is induced to combust (viz., undergo re-burning). With this arrangement the regeneration is conducted for either a predetermined time or distance.
However, this arrangement has suffered from the drawback that trap regeneration sometimes does not proceed as expected.
One reason for this comes in that the rate of particulate accumulation varies markedly with the manner in which the driver operates the engine, the altitude, engine load, engine and ambient temperature, fuel pump settings, age of the engine, etc. Accordingly, if the regeneration is induced at regular intervals (based on either time or distance) it sometimes occurs that an abnormally large amount of particulate matter accumulates between regeneration.
This leads to a serious problem that the amount of accumulated particulate matter sometimes exceeds a critical level. Accordingly, during a regeneration, overly intense combustion tends to occur. This raises the temperature of the trap beyond its thermal limits and induces a damaging melt-down or like.
In the event that the frequency of the regenerations is increased to ensure that a critical amount combustible matter cannot accumulate, the frequent arbitrary closing of the throttle valve deteriorates both engine performance and fuel economy.
Further reasons for the unstable trap regeneration come in that during a regeneration, the exhaust gas temperature varies with the ambient air pressure and other driving conditions, and as a result of the reduced air induction which raises the exhaust temperature, the amount of particulate which is contained in the exhaust gases increases. Accordingly, if the exhaust gas temperature is not raised to the levels expected, the regeneration efficiency drops off and it sometimes occurs that the amount of accumulation during the re-generation, at least in part, replaces that actually being combusted and the amount of combustible particulate matter retained in the trap 3 immediately following the termination of the regeneration can be substantial and/or essentially the same as initially contained therein.
This also leads to the problem that the amount of accumulated particulate matter sometimes exceeds a critical level and results in the above mentioned damagingly intense combustion.
In view of the above, it has also been proposed to monitor the pressure differential which exists across the trap and to trigger the regeneration upon a given back pressure developing. However, it has been found that the accumulation of incombustible matter such as metal oxides (resulting from the combustion of additive containing lubricants etc) in the trap render this technique of determining the amount of combustible particulate matter unreliable.